Planet ad absurdum

Mercury will tomorrow be farthest out from the Sun (to the west – the right as seen from our northern hemisphere). It’s trying to make itself visible in the pre-sunrise twilight.


But it doesn’t try very hard, and this is really only to show you why this “greatest elongation” of Mercury (the first of three morning ones in the year) is unfavorable.  To see it you’ll have to try harder.

The diagram shows that although the little planet has swung out to a large angular distance from the Sun (26.7 degrees, almost the largest elongation it will reach in the whole year), it is out at a low angle to the horizon. This is because it is in the part of its orbit which slopes south from where the Sun is. (Put another way, its declination is more southerly than the Sun’s; it is farther below the celestial equator.)

For people in South Africa or Australia it’s very different: Mercury’s southerliness favors them and they will see it high in the dawn. The tilted and eccentric geometry of the innermost planet’s orbit keeps cheating us in northern lands, as has to be explained every year in the Astronomical Calendar‘s Mercury section.

For better opportunities to glimpse Mercury, we have to wait till the evenings of around May 6, and the mornings of around October 16.

I decided to switch back to drawing these diagrams for the eastern American longitude of 75 degrees west (instead of the Greenwich longitude of 0). This may help a bit in the explanation of the clock times. The absolute time of the picture, about 11 hours Universal Time, is 11 AM in Britain, 6 AM in eastern North America, 3 AM on the west coast. But the diagram is true for the time of day – the time relative to sunrise – for all of those and other longitudes. That is, the stars will be in the same places and so will Mercury, almost exactly. (Only if the fast-moving Moon were in the picture would its changing positions have to be explained.)

The instant of the Feb. 24 “greatest elongation” event is not 11 UT but 16 UT, about 5 hours after the time of our picture. But this makes no appreciable difference, because at such an event the planet is hovering essentially still in relation to the Sun. We are looking at it as it moves directly away from us – looking at it along a tangent to its orbit. So it doesn’t matter whether it’s Britain or New York or California you’re looking from about 6 AM.


Differences of latitude make a difference: if you’re farther north than 40, Mercury will be lower. If you’re at the north pole, Mercury will be below the horizon, indeed below the Sun. Reductio ad absurdum is often a trick toward understanding.

6 thoughts on “Planet ad absurdum”

  1. Here on the other side of North America (compared to Anthony in San Francisco), in Virginia, I have managed to spot Mercury three times during this morning apparition, with the aid of binoculars, through the bare trees from our bedroom window. This despite our suddenly very snowy and cold winter this February. Being amid the dim “watery” constellations now, there is no mistaking it for anything else!

  2. The conjunction of the Moon, Venus, and Mars on Friday night, with Uranus in the background and Jupiter looking on from the east, gave me a vivid opportunity to think about where we and our siblings are in the solar system, and how everything is moving. On Friday night I took a pair of binoculars and a tripod to a hilltop park near my home to share the view with passersby, and then on Saturday my astronomy club had a public star party, so that was another opportunity to help people get to know the neighborhood.

    I’ve seen Mercury twice during his current morning apparition, and even was able to point him out to a couple of young women who had gotten up early to watch the Sun rise, but Mercury is pretty elusive. I wouldn’t call him absurd, because I don’t want to give offense.

    1. Hermes (Mercury) was according to Greek myth a mischievous godling: just out of the cradle, he got into absurd trouble for stealing Apollo’s cattle.

      1. True enough, but he’s older now, and he still has a wicked sense of humor. I try to stay on his good side.

        1. Have you read Marie Phillips’ novel, _Gods Behaving Badly_? The Greek pantheon are living in much reduced circumstances in a contemporary London townhouse. It’s a fun read.

          By the way, with remarkably clear weather here in San Francisco, I just saw Mercury again, naked eye, half an hour before sunrise. This was the conclusion of a good night of solar system bingo. Venus in bright twilight showed her gibbous disk through a small telescope with a polarizing filter. Venus and Mars slowly separating after their rendezvous. Uranus in binoculars. I showed the waxing crescent Moon to passersby in front of my home. A sweet man on his way to take an ESL test told me of the dark sky where he grew up in Mexico, and we traded names of asterisms in Spanish and English. I watched from the back yard as Io emerged from Jupiter’s shadow. Then a few hours sleep before taking the little telescope around the corner to see Saturn’s shadow on the rings, and finally Mercury. I’ll be tired at work today, but it was worth it.

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